In erecting a building having more than one floor level there usually arises a need for a space to be left in a floor typically to install stairs between that and another floor. Such an opening needs to be protected in some way to prevent accidents arising from people or articles inadvertently dropping through the space. However the form of protection used should preferably not lead to access problems for acceptable use. A number of proposals have been made to enable such a space to be protected. However these do result in limitation, if not prevention, of the use of the space for passage of people or transfer of articles.